Receiving apparatus for radio beacon systems



July 30, 1946. w. .1. o'BRn-:N 2,404,810 RECEIVING APPARATUS FOR RADIO BEACC'JN SYSTEMS Filed'Deo. 23, 1941 2 Sheets-Sheet l li i TE- Z55 sa illl'illmil Alle flu/rex I EORNE E. z

July 30, 1946. w. J. o'BRlEN RECEIVING APPARATUS FOR RADIO BEACON SYSTEMS Filed Dec.- 23, A1941 2 Sheets-Sheet ,2

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atentc July 30, 1946 RECEIVING APPARATUS FOR RADIO BEACON SYSTEMS William J. OBrlen, Los Angeles, Calif., assignor to The Decca Record Company, Limite Lon- don, England, a corporation of Great Britain Application December 23, 1941, Serial No. 424,159

4 Claims. 1

My invention relates to radio beacon systems and has particular reference to a radio receiving and indicating apparatus which finds particular utility when employed with an equisignal type of beacon system.

Equi-signal beacon systems operate to generate a field strength pattern which includes a line along which the signal intensity of'two groups of radio transmitters are equal, which line is the course" deiined by the beacon and along which it is intended that the aircraft or other vehicle be maneuvered.

The signals from the two groups of transmitters are rendered distinguishable either bymodulating the signals at different audible frequencies or by keying the two groups of transmitters off and on alternately and in a distinguishable time sequence, the presently employed arrange-v ment being that in which one group of transmitters is keyed with the International Morse code signal for A while the other is alternately keyed N. In either event the on course position of the vehicle is indicated by the reception at the vehicle of signals of equal intensity from the two groups of transmitters and an oil course location is indicated by the predominance of one signal over the other.

In both of these types of Aequi-signal beacon systems the signals are perceived audibly with the result that extraneous noises such as static and other radio interference tends to make it more diiilcult to distinguish between the signals. Audible perception of the signals requires. that the pilot of the aircraft or other vehicle wear headphones or similar devices and also requires the pilot to consciously direct his attention to distinguishing the signals with the result that the pilot's freedom of movement is impaired and his attention is diverted from the control and from the navigation instruments.

The desirability of providing for a visual perception of the beacon signals has` long been recognized and while attempts have in the past been made to realize this desiratum, no one prior to my invention has provided a satisfactorily operating device.- 'I'he modulating of the signals at different audible frequencies represents one such attempt, these different frequencies being applied to vibrating reeds in the belief that a comparison of the amplitudes of the vibration of the reeds would provide an indication of the relative strengths of the signals. This expedient was not satisfactory because of the necessity of providing suitable audio frequency generating and modulating mechanism for the beacon trans- 2 mittersl and because of the liability of the reeds gettingout of order through mal-adjustment or injury, the reeds of necessity being very delicate and lacking in mechanical strength. Furthermore, it was found that the indication given was so coarse as to make it extremely difficult for a pilot to keep his craft on course without having recourse to earphones, whereupon all of the hoped for advantages were lost.

It is, therefore, an object of my invention to provide a receiving,A apparatus for use with radio beacon systems of the equi-signal ,type

'which overcomes the above noted disadvantages by providing a visual indication ofthe location of a vehicle equipped with the apparatus relative to a course defined by the beacon system.

It is also an object of my invention to provide a receiving apparatus of the character set forth in the preceding paragraph in which the indicating portion of the apparatus is highly sensitive and operates to positively indicatevisually small departures of the vehicle from the dened course. v

It is an additional object of my invention to provide an apparatus of the character set forth hereinbefore in which the receivedv signal is rectiiied and the polarity of the. peak of the resulting signal is employed to indicate the position of: the vehicle relative to the course.

It is a still further object of myinvention to provide an apparatus of the character hereinbefore referred to in which the received signal is rectied to provide an alternating current of complexl wave form and which also includes means for comparing the phase relation between the fundamental and the second harmonic inimum peak current.

It is additionally an object of my invention to provide a method of visually indicating the position of a vehicle with respect to a course defined by a radio beacon of the equi-signal type which consists in rectifying the beacon signals to produce a' current of complex wave form a1- ternating in accordance with the keying of the beacon transmitters and determining the phase A the "on course zone representing the areas inwhich one of the beacon signals predominates in intensity over the other beacon signal;

Fig. 2 .is a graph illustrating the way in which the relation between the intensities of the two groups of beacon signals varies with changes of the position of the vehicle with respect to the "on course line denned by the beacon;

Fig. 3 comprises-a group of charts or graphs indicating the character of the received beacon signals at various stages in the progress of the signals through the receiving apparatus of my invention;

Fig. 4 comprises a series of graphs or charts indicating the character of the signals at points in the receiving apparatus subsequent to those represented in Fig. 3;

Fig. 5 is a wiring diagram schematically illustrating one form of receiving apparatus constructed in accordance with my invention;

Fig. `6 is a schematic wiring diagram illustrating a modified form of receiving apparatus;

Fig. 7 is a graph illustrating the manner in which the rectied beacon signals can be approxiof the signals is caused to be radiated for a relatively short time, whereas, the other is caused plotted as ordinates and time is plotted as mately resolved into a fundamental sine wave and a sine wave of double frequency; and

Fig. 8 comprises a series of charts or graphs portraying the action of the indicating portion of the apparatus embodied in the modification of my invention which is illustrated in Fig. 6.

Referring to the drawings, I have illustrated in Fig. 1 an aeroplane P or other vehicle as being guided outwardly along an "on course zone which is bounded in Fig. 1 by the lines l and 2. This zone comprises that area immediately adjacent a line of equal signal intensity with respect to independent radio frequency signals emanated A from two groups of radio transmitters.

The type of radio beacon transmitting equipment employed for producing an equi-signal on course zone is well known and forms no part of the present invention. For a detailed explanation of such a radio beacon system reference may be had to the Bureau of Standards Journal of Research for January, 1933, vol. 10, page 7. Such a system operates to produce within the zone bounded by the lines I and 2 separate radio frequency signals of equal intensity, whereas, in the zone indicated generally by the reference character 3 and lying to the left of the line i, one of the beacon signals will predominate in intensity over the other, whereas, in the zone indiq cated generally by the reference character i and lying to the right of the line 2, the other radio beacon signal will predominate over the firstmentioned signal.

In accordance with" my invention the radio beacon transmitters are preferably keyed o and on to provide for the distinguishing of one set of radio frequency signals from the other set. This keying is so arranged that one of the radio frequency signals is present during the time the other is absent and vice versa. Furthermore, one

abscissa. That portion of the chart lying between the origin and the wavy line 5 represents the signals which are received at the plane P when that plane is in the oi course zone 3.

It will be noted that the signals received at the plane P comprise high intensity peaks A of relatively short duration and low intensity signals B of relatively long duration. The peak vsignals A will hereinafter be referred to as being the A signals, whereas, the signals of longer duration will be hereinafter referred to as the B signals.

As the plane P moves nearer and nearer to the on course zone, the intensity of the A signals will decrease while the intensity of the B signals will increase until these A and B signals are of equal intensity when the plane is in the position illustrated in Fig. 1. When this condition obtains, the signals received at the plane are equal and indistinguishable as is indicated in Fig. 2 in that portion lying between the wavy lines 5 and ii. If, on the other hand, the plane P is positioned to the right of the on course zone and in the zone d, the relative intensities of the signals will be reversed from that previously described with the result that the B signals will predominate in intensity over the A signals as is illustrated in Fig. 2 by that portion of the figure disposed to the right of the wavy line 6.

The receiving apparatus comprising the preferred form ofI my invention is illustrated diagrammatically in Fig. 5 as comprising an ordinary radio frequency receiver and amplifier indicated generally by the rectangle bearing the reference character 1, this receiver being adapted to pick' up the A and B signals as by means of a suitable antenna 8.

The amplified output of the receiver 'l is coupled by means of a coupling transformer 9 including a tuned primary winding I0 and a tuned secondary ll to a grid controlled rectifier tube l2. The signal voltage developed across the winding ll is applied between the plate and cathode of the tube l2, the cathode return circuit including a resistance la shunted by a condenser M.

The grid to cathode circuit of the tube l2 is preferably coupled as by means of a suitable coupling transformer l5 to a radio frequency oscillator of any suitable type represented diagram-` matically by the rectangle I6. The oscillator is adjusted to produce a radio frequency current which differs from the frequency of the signals A and B by an amount adapted to produce a beat frequency signal which is preferably, although not necessarily, within the audible range.

Since the high frequency produced by the oscillator I6 is applied between the grid and the cathode of' the tube l2 and since the received A and B signals are applied between the plate and cathode of the' tube 2, the tube l2 will operate to rectify the voltage appearing across the winding il only at such time as the grid and plate are both positive with respect to the cathode. The tube i2 will accordingly produce in the resistance I3 a pulsating direct current, which pulsations have a frequency equal to the difference in frequency between the signal produced by the oscillator it and the frequency of the A and B signals. If desired, the negative potential produced on the plate of the tube I 2 may be used for the purpose of the automatic volume control in the receiver 1, as by connecting the plate circuit for the tube I2 to the grid circuits of the various tubes employed in the amplifier 1 through an automatic volume control bus AVC.

The resistance I 3 preferably comprises a potentiometer, the arm portion of. which is connected as by means oi.' a conductor I1 through' a blocking condenser I8 to the grid of an amplider tube I9. The grid of the tube I9 is maintained at a direct current ground .potential as by connecting the grid to ground through a grid resistance 29. 'I'he tube I9 is employed as a' voltage amplifier and is coupled through a suitable coupling transformer 2| to the input circuit of a power ampliiier tube 22 whose output appears across the secondary of an output transformer 23.

In Fig. the heater circuits have been omitted as have the plate supply and screen supply circuits for the tubes I9 and 22 since these circuits are well known in the art and form no part of the present invention. l

The secondary of the transformer 23 is preferably grounded at a mid tap as by a conductor 24, whereas, the ends of the windings may be connected as by means of conductors 25 and 26 to headphones or other suitable audio frequency responsive means 21 if it is desired to permit the pilot of the plane P or other vehicle to perceive the signals A and B audibly.

One end terminal of the secondary of -the transf former 23 is preferably connected as by means"- of a conductor 28 to a half wave rectifier 29, whereas, the other end terminal is connected as by means of a conductor 30 to a half wave rectiiier 3i, the cathodes of the rectifiers 29 and 3| being interconnected as by means of a, conductor 32 which is in turn connected to ground through a resistance 33.

The voltage which may be produced across the resistance 33 may also be transmitted to a resistance 34 through a low pass lter including series connected inductances 35 and 36 and shunt connected condensers 31 and 38. Such voltage as may appear across the resistance 34 is applied to the primary portion of the winding of an autotransformer 39 through a condenser 49, the mid tap of the auto-transformer winding being grounded as by means of a conductor 4I.

The end terminals of the auto-transformerv winding are connected, respectively, to the anodes of half wave rectiiiers 42 and 43, the cathodes of these rectiflers being connected to ground, respectively, through resistances 44 and 45. The resistances 44 and 45 are preferably shunted by condensers 46 and 41, respectively. The cathodes of the rectiers -42 and 43 are furthermore interconnected by a galvanometer 48 which is 'preferably of the zero center. type and of the current responsive type ance.

The manner in which the above described apparatus operates to provide a visual indication of the location of the plane P with respect to the on cours zone may best be had by assuming certain operating conditions and describing the operation of the apparatus in connection with the explanatory drawings, Figs. 3 and 4. If it be assumed that the plane P is to the left of the "on course zone and in the area indicated generally by the reference character 3, the signais received at the plane P by the antenna 8 will be such as are represented by that portion having a very low internal resist-4 of Fig. 2 lying between the origin and the wavy line 5; that is, the A signals will predominate in intensity over the B signals. The output of the receiver 1 willI therefore, correspondingly uctuate and these fluctuations will likewise appear in the intermediate or audible beat frequency which is amplified by the ampliiier tubes I9 and 22. The voltage appearing across the secondary of the output transformer 23 will, therefore, be such as that ,represented by the sine curve 49 in Fig. 3.

It will be noted that this curve varies in amplitude in accordance with the relative amplitudes of the A and B signals.V The curve 49, therefore,

represents the voltage which is applied to the represents the voltage which appears across the j resistance 33.

The filter vcomprising the inductances 35--36, the condensers 81-33 and the resistance 34 operates to exclude thel high frequency component of the pulsating voltage represented by-the curve 59. 'The condenser 49 serves to isolate the winding of the auto-transformer 39 with respect to direct current with the result that there is applied to the primary portion of this winding an alternating voltage such as-that illustrated by the'curve 5| in Fig. 3.

Attention is directed to the fact that by eliminating the high frequency component and the direct current component from the voltage which is appliedto the primary portion of the winding of the auto-transformer 38, the axis 52 of zero 1:.

voltage is automatically caused to assume a position -in which the area embraced between the positive 'portions of the alternating current wave 5I will have a peak value considerably exceedingv the pea'kvalue of the negative half cycle. This voltage is accordingly applied to the rectifier 42, being the voltage which is applied across the primary portion of the winding ofthe autotransformer 39. The voltage which is developed by the other half of the winding of the autotransformer 39 is, of course, 180 out of phase with respect to the voltage Awhich is applied to the primary portion of this winding. This .out of phase voltage may accordingly be represented by the curve 53 of Fig. 4. It will be noted that the voltage which is applied across the rectifier 42 and resistance 44 is the voltage represented the B signal is of relaby the curve 5|, whereas, the voltage which is applied across the rectifier 43 and its resistance 45 is the voltage which is represented by the curve 53.

The rectier 42 will pass current during the time the plate thereof is positive with respect to the cathode and this current will flow through the resistance 44 tending to make the catliode of the rectifier 42 negative with respect to ground.

The ohmic value of the resistance 44 and the capacity of the condenser. is so chosen as to ascesi@ maintain the voltage of the cathode of the rectier 42 substantially constant with respect to ground. That is to say, during the relatively short intervals that the high positive peak voltage of the c'urve 5I is applied to the rectifier 42, the voltage which appears across the resistance 44 and condenser 46 is of a corresponding magnitude and may be represented by the short horizontal line portions 54 of the lower of the two curves illustrated in Fig. 4. During the following negative half cycle, the rectifier will, oi course.'

pass no current with the result that the condenser 45 will discharge its stored energy through the resistance 44 and the voltage measured between the cathode of the rectifier 42 and ground will gradually diminish as is represented by the sloping line 55 in Fig. 4. As soon as the plate of the rectifier 42 again swings pdsitive, the voltage across the resistance 44 will immediately rise to the straight line value 54.

It will be recalled that the cathode of the recti- `fier 42 is directly connected to the cathode of the rectifier 43 through the low resistance galvanometer 48 so that the cathodes of the two rectiers are maintained at substantially the same potential with respect to ground. This negative potential will exceed the positive peak value of the voltage which is applied to the plate of the rectifier 43 as may be readily seen by comparing the amplitude of the curve 54-55 with theamplitude of the positive half cycle portions of the curve 53 in Fig. 4. Thus the rectifier 43 will be blocked and willpass no current.

Similarly, the galvanometer 48 serves to connect the resistance condenser combination 45--41 in parallel with the resistance condenser combination 44-46 so that the current which ows from the plate to the cathode of the rectifier 42 will divide, half fiowing through the resistance 44 and half flowing through the galvanometer 48 and thence through the resistance 45. This current owing through the galvanometer 48 will cause the needle thereof to deflect in one direction, to the right for example, from its Zero center position and this deection may be used as an indication of the fact that the signals as picked up by the antenna are in accordance with the assumptions upon which the preceding description has been based; namely, that the plane or vehicle P is to the left of the on course zone and in the area 3 so that the A signal predominates over the B signal.

Assuming now that the plane P crosses the on course zone and shifts to the right of the desired course so as to be positioned within the area, 4. Reference to Fig. 2 will indicate that the B signals will predominate over the A signals with the result that the voltage which is applied to the primary portion of the Winding of the auto-transformer 39 will be 180 out of phase with that represented by the curve 5| in Fig. 3 and will instead correspond t0 the curve 53 of Fig. 4.

Similarly, the voltages applied to the rectiflers 42 and 43 will be reversed from that described in the preceding paragraphs with the result that the rectifier 42 will be blocked, the rectifier 43 will pass current, and current will flow from the rectifier 43 and through th'e galvanometer 4B and thence through the resistance 44 to ground. The direction of current now through the gaivanometer is thus reversed and its deflection from zero center will likewise be to the left or opposite to that resulting from the conditions assumed in the preceding description. It will thus be seen that the position oi the galvanometer needle to the right or to the left of its zero center will correspondingly indicate the direction in which the vehicle must be moved to bring it back to the desired course.

Similarly, if the vehicle is on;course," the galvanometer will read zero. This is for the reason that the rectification of the A and B signals eected by the rectifiers 29 and 3| will provide across the resistance 34 a pure direct current so that no alternating voltage will be applied to the auto-transformer 39. Consequently no current can ow in the circuits associated with the galvanometer '43 and the galvanometer will, of course, indicate zero or its center position.

I have illustrated .in Fig. 6 a modified form of apparatus which may be employed to provide a visual indication of whether the A signal or the B signal predominates in intensity. In Fig. 6 the antenna 3 is illustrated as being connected to the primary of an input transformer 56, the secondary of which is connected in the grid circuit of a radio frequency amplifying tube 51. The output of this tube is coupled to an oscillator and mixer tube 58 which functions to mix the signals picked up by the antenna 8 with a radio frequency signal of slightly different frequency produced by the oscillator portion of the tube 58 to thereby produce an output of intermediate frequency equal to the difference in frequency be4 tween the oscillator signal and the A-B signals.

This intermediate frequency is amplified by intermediate frequency amplifying tubes 59 and 60 and by a power amplifier 6|. The output of the power amplifier 6| is connected to a rectier 62, in the cathode circuit of which is included a resistance 63 across which appears the rectined voltage represented by the curve 50ct Fig. 3 previously discussed. The intermediate frequency component of this voltage is filtered. out by means of condensers 64-65 and a resistance 66 and the direct current voltage is blocked by a blocking condenser 61. The condenser 61 is connected to a conductor 68 which interconnects the grids of amplifying tubes 69 and 10, these grids being also maintained at a direct ground potential by a ground connection through a grid resistance ll. The voltage which is thus applied to thev grids of the tubes 69 and 'I0 is the alternatiig voltage represented by the curve 5I in Fig.

Before describing the operation of the tubes 69 and 'l0 and the circuits associated therewith, attention is directed to Fig. 7 where the grid voltage curve 5I is reproduced to a somewhat larger scale and indicated by the heavy line 5l. It can be shown that the wave form of the curve 5l comprises a fundamental sine wave having the same frequency as the curve 5|' upon which is superimposed an infinite number of sine wave harmonics in progressively diminishing amplitudes.

Of these various components, the fundamental and the second harmonic appear to the greatest amplitude. The light lines l2 and i3 in Fig. 7 represent such a fundamental and second har-a monic, whereas, the dotted line 'I4 represents the net sum of the fundamental l2 and. its secondis coupled through a coupling circuit 11 to a power amplifier tube 18 to comprise an ainplifyingchari-v nel adapted to passend amplify the fundamen- Y tal 12 and to reject frequencies both higher and lower than this fundamental.

The input circuits for the tubes 69 and 10 are identical in principles as are the coupling devices Y 15 and 11 and hence it is deemed sufiicient to, describe in vdetail but one of each of these circuits.

The cathode of the tube 69 is connected to ground through a cathode resistance 19 which is shunted by a by-pass condenser 80, the resistance 19 being employed in the conventional manner to provide the desired grid bias for the tube. The

. cathode circuit for the tube 69 also includes a'relatively high inductance 80" to suppress static and other undesirable high frequency signals which may appear in the input circuit ofthe 'tube 69. The output of the tube 69 is coupled to-the cour pling circuit 15 as by means of a. plate blocking condenser 8|.

The coupling circuit 15 embodies a transformer which includes a primary 82 and a secondary winding 83, the lowerends of which are interconnected as by a conductor 84 and connected to ground through a condenser 85. The upper end of the primary 82 is connected to the output of the tube 69 through the condenser 8|, whereas, the upper end of the winding 83 is connectedto the grid of the tube 16 through a grid bias cell 86 adapted to provide the desired value of negative grid bias on the tube 16.

A xed tuning condenser 81 is connected between the upper terminal f the winding 83 and ground, whereas, a similar condenser 88 is connected between the upper terminal of the winding 82 and ground. The winding 82 is tapped as at 89 and connected to ground through a' resistance 90, the tap 89 beinglocated at a point of zero voltage with respect to the alternating voltage appearing across .the winding 82. n

The constants of the above described circuits are so chosen that the inductance of the winding' 82 shunted by the series connected condensers 88 and 85 is tuned to the frequency of the secon harmonic 13 of Fig. '1. Similarly, the series connected condensers 85 and 81 are so chosen with respect to the inductance of the winding 83 as to tune that circuit tothe frequency of the second harmonic. The capacity of the condenser 85 is so chosen with respect to the mutual inductance of the windings82 and 88 as to provide an extremely low impedance path to ground for signals having a frequency one-half that of the harmonic 13. The coupling circuit 15 thus serves to pass to the tube 16 thel signal comprising the second harmonic 13 While serving at the same time to denitely filter out and exclude from the tube 16 the fundamental represented by the curve 12 in Fig. '7.

The resistance 90 serves to connect the winding 83 to ground as regards direct current so as to permit the cell 86 to apply the requisite grid bias to the tube 16. By connecting the resistance 90 to the zero voltage point 89 on the winding 82, the

. l0' resistance 90 does not ing of the coupling circuit.. y

The Vsame considerations are involved "in the choice of the constants for the coupling circuit 11 with the exception that this circuit is tuned to Pass the fundamental 12 of Fig. '7 and to defin itely lreject signals Vhaving half that frequency. 'I'he second harmonic is substantially excluded from the tube'18 by the tuning effect of the coupling circuit" and this selectivitymay be furt'le cathode" circuit inductance 80 for the tube I'he output ofthe power amplier 16.is con.

transformer 92, which transformer is provided -with two electricallyI separatedsecondary windings 93 and 94. The outermost terminals of the windings 93 and 94 are connected, respectively, to the plates of the grid controlled rectifier tubes 95 and 96, the grids of these tubes being connected in parallelas by a conductor 91 and coupled to the output .of the amplifier tube 18 as by v means of a condenserl 98 and resistance 99.l

The cathodes `of the rectifiers 95 and 96 are interconnected and connected to ground through the cathode bias resistor |00 which is 'preferably l shunted by a by-pass condenser |0I. The winding 93 is preferably tuned to the frequency of the secohd harmonic 13 as by a condenser |02 and the inner end of this winding is connected as by means of a conductor |03 through a resistance |04 to ground, this resistance being shunted by a condenser |05. In a similar manner the winding 94 is tuned by a condenser |06 and the inner end of the winding is connected by a conductor |01 to ground through a resistance |08 which is shunted by a by-pass condenser |89.

The conductors |03'and |01 are,'respectively,. l

by interconnecting lthe cathodes of these tubes.

and connecting the interconnected cathodes to a tap I1 of a potentiometer comprising lresistances I8 and ||9 connected between the source of plate potential and ground'.

The manner in which the above described apparatus operates may best be understood by assuming certain operating conditions and describa ing the operation of the device with particular reference to Figs. 3, '7 and 8. I

As was pointed out in connection with the first described form of my invention, the signal which is picked up by the antenna 8, amplified by theamplifler 51, converted to .an intermediate frequency by the mixer tube 59 and amplified by the amplifier tubes 59, and 6| is represented by the'curve 49 of Fig. 3 if it be as'sumedrthat the plane or vehicle P is to the left of the desired course and in the off course area 3. Upon rec.. tication of the signal 49 by the rectifier 62, there is developed across the resistance 63 a pulsating direct potential such asthat represented by the f curve 50 in Fig, 3. The filter network 64-65-66 and the condenser 61 serve to eliminate the intermediate frequency component and to shift the axis of zero voltage so as to apply to the gridsf in any way affect the tunther increased by proper choiceof inductance for nected to a primary winding 9| of an output 1l 10 and 11 into the fundamental 12 and the harmonie 13 so that there is applied between the plate and cathode of thetube 90 an amplied second harmonic 13. This plate to cathode voltage of the tube 95 is represented in Fig. 8 by the sine curve |20 shown in solid lines in that figure.

Since the transformer secondary winding 93 is opposed to the winding 93, there will-be applied between the plate and the cathode of the tube 90 a voltage similar to the voltage |20 but being 180 out of phase with respect thereto. This voltage which appears between the plate and cathode of the tube 96 is represented by the curve |2| shown in solid lines in Fig. 8.

The fundamental 12 of Fig, 'I is amplied by the tubes 10 and 18 and is applied through the coupling 90-99 to the grids of both of the tubes 95 and 96. This grid voltage is represented by the curve |22 of Fig. 8. 'I'he bias resistance |00 for the tubes 95 and 90 is so chosen as to shift the axis of the curve 22 an amount represented by the dimension line |23 in Fig. 8 sufcient to make the lateral width of that portion of the curve 22 which is positive with respect to ground equal to the lateral width of the curves |20 and |2| at the axis of these curves.

It will be noted that undei` the assumed conditions; namely, that the vehicle P is in the oif course area 3 so that the signal A predominates in intensity over the signal B, the phase relation between the fundamental 'l2 and the harmonic 13 will be as shown in Fig. '7. This phase relationship is reproduced in the curves |20, |2| and |22 of Fig. 8. By comparing these curves it will be noted that every other positive half cycle of the curve |20 occurs during the same time as d oes positive portions of the curve |22. To facilitate this comparison the positive portions of the grid voltage curve |22 have been shaded and those positive loops of the curve |20 which occur at the same instant as do the periods of positive grid voltage have been similarly shaded. Thus, the plate of the tube 95 is positive with respect to its cathode at the same time that the grid of this tube is also positive with respect to the cathode. Thus the tube 95 is capable of rectifying and a corresponding direct current, therefore, ows through the resistance |04 which results in the conductor |03 having a negative potential with respect to ground.

The condenser |05 which is connected in shunt relation to the resistance l@ is preferably provided with a capacity suiil'ciently large with respect to the resistance |09 as to maintain the voltage across the resistance |00 during the time between the periodic moments of rectiiication of the tube 05. The direct potential which is thus applied to conductor |03 vis such as that represented by the curve |20 in Fig. 8.

By comparing curves |2| and |22, it will be noted that at no time does the grid and plate of the tube 96 have positive potentials applied thereto at identical times with the result that the tube .B0-will not perform any rectifying function and no voltage will be developed across the resistance |08. Thus the conductor |01 remains at ground potential.

The tubes M2 and-||3 are normally biased to operate on the straight portion of the grid voltage-plate current curve, this grid bias voltage being represented in Fig. 8 by the dimension line |255, and the resistances i i and i0 are so chosen that the voltage of both of the plates with respect to ground will be equal when the grids of is connected as the balance responsive element.

Thus the galvanometer I4 will always indicate n center zero when the grids of both of the tubes ||2 and i3 are maintained at ground potential.

Under the assumed conditions, however, the grid of the tube ||3 is maintained at ground potential by virtue of its connection to the conductor |01 while the grid o! thetube |I2 will be depressed to a voltage which is negative with respect to ground by virtue oi its connection to the conductor |03. The application of the negative voltage to the grid of tube ||2 will throw the Wheatstone bridge out of balance and this out of balance condition will be indicated by B corresponding deflection of the galvanometer Ill. This deflection ofthe galvanometer will,v therefore, serve to indicate that the plane or vehicle P is to one side of the on course zone and in the oil' course area 3.

By having reference to Fig. 7 it will be observed that should the B signal predominate over the A signal so that the voltage which is applied to the grids of the tubes 99 and 10 is characterized by a low positive peak and a high negative peak, the analysis o1' this voltage into the fundamental 12 and the harmonic 13 results in the phase of the harmonic 13 being precisely reversed; that is. with the negative peaks at the points now occupied in Fig. 'I by the positive peaks. This change will also reverse the phase of the voltage which is applied between the plate and cathode of the tubes and 96, this phase reversal being indicated in Fig. 8 by the dotted lines |20' and |2|.

' Comparing the grid voltage curve |22 with the dotted plate Voltage curves |20 and |2| will show that the tube 95 will never be in a condition to rectify the signal applied thereto, whereas, the grid and plate of the tube 9B will periodically become positive at the same instant as is represented by the shaded portions of the dotted curve |2|' so that this tube may rectify. 'I'he result is that the grid of the indicatortube ||2 is maintained at ground potential, whereas, the grid of the tube ||3 is depressed to a negative value with respect to ground by virtue of its connection to the conductor |01 embodied in the cathode circuit to the tube 90. This likewise produces an unbalanced condition of the Wheatstone bridge circuit but the circuit is unbalanced in the opposite direction from that previously described so that in the instant case the galvanometer deflection is in theopposite direction and serves to indicate that the plane or vehicle P has wandered in the opposite direction from the "on course line and into the off course zone 4.

'I'he galvanometer |41, of course, indicates cen ter zero when the plane or vehicle P is on course for the reason that a pure direct potential is developed across the resistance 63, which potential is blocked by the condenser 61 so that no alternating voltage is applied to the grids of the tubes 69 and 70. Thus, no control voltage is developed by the rectifiers 95 and 96 and the tubes ||2 and H3 are maintained in their normal operating .condition corresponding to a balanced condition of the Wheatstone bridge.

From the foregoing it will be observed that I have provided a novel receiving and indicating apparatus for use with radio beacon systems of the equi-signal type, which apparatus is particularly adapted to provide a sensitive and dependable visual indication of the location of the aircraft or other vehicle with respect to the course which is defined by the radio b'eacon system.

It will be noted that the rst described modication of my invention operates to identify which side of the course the vehicle may be located by determining the polarity of the maximum voltage resulting from a rectiiication of the beacon signals. Conversely, the second described modification of my invention functions to analyze the voltage resulting from the rectification of the beacon signals into a fundamental and second harmonic and indicates which side of the course the plane or vehicle is situated by indicating the phase relation between this fundamental and its harmonic. v

Attention is directed to the fact that the device of my invention is adapted for use with substantially any equi-signal radio beacon system, the only requirement being that the keying of the transmitters employed in such system be such that the time of transmission of one of the signals exceeds the time of transmission of the other. By a relatively small and inexpensive change, the presently commercial radio beacon systems of the A-N type could be readily adapted for use with the radio beacon receiver and indicator of my invention, this small change comprising merely an adjustment of the keying sequence to provide for the difference in time of operation above mentioned.

While I have shown and described the preferred embodiment of my invention, I do not desire to be limited to any of the details of construction shown or described herein, except as dened in the appended claims.

I claim:

l. The method of visually indicating the prel of like frequency which are produced alternately in a recurring time cycle and for unequal periods 'in each cycle, the combination of: a radio frequency amplifier for receiving and amplifying said signals; rectifying means for rectifying said' amplied signals to produce a direct potential which pulsates in accordance `with the alternate production of said signals; filter means for separating the alternating potential and direct potential components of said pulsating potential; separate low frequency amplifying channels, one tuned to pass and amplify only the ,sine wave fundamental of said alternating potential component and the other tuned to pass and amplify frequency amplifier for receiving and amplifying i said signals; lrectifying means for rectifying said amplied signals to produce a direct potential which pulsates in accordance with the alternate production of said signals; filter means for separating the alternating potential and direct potential components of said pulsating potential; separate low frequency amplifying channels, one tuned tovpass and amplify only the sine wave fundamental of said alternating potential component and the other Ituned to pass and amplify only the sine wave second harmonic of said alternating potential component; a pair of grid controlled rectifiers connected in a full wave rectifying circuit with said other amplifying channel and having separate load resistances; means conf, necting the output of said one amplifying chanproduction of said signals, separating the direct potential component from the alternating potential component of said pulsating potential, ascera taining the polarity of the maximum potential peak of said alternating potential component by separating from said alternating potential component the fundamental and second harmonic sine wave components, and determining the multiple phase relation between said sine wave comy ponents.

2. In a device for visually indicating the predominance in intensity of one of two radio signals nel to the grids of said grid controlled rectiflers; and means for measuring the direct potential across said load resistances.

4. A device for visually indicating the polarity t monic.

WILLIAM J. OBRIEN. 

